Signaling through muscarinic acetylcholine receptors (mAChRs) in the basolateral (BL) nucleus of the amygdala plays an essential role in the formation and extinction of emotional memory. Accordingly, in all mammals, including humans, the BL receives more robust cholinergic innervation than any other target of the basal forebrain. Behavioral studies have demonstrated that activation of mAChRs in BL enhances fear memories, whereas blockade of these receptors prevents memory consolidation. Moreover, Alzheimer's disease as well as neuropsychiatric disorders such as schizophrenia, which are commonly associated with emotional disturbances, are thought to result, at least in part, from abnormal cholinergic transmission. Indeed, in Alzheimer's patients the extent of degeneration of cholinergic input to BL is correlated with their impairment of emotional event memory. These findings clearly suggest that therapeutic modulation of mAChR-mediated mechanisms in the BL could be important for treating a number of major neuropsychiatric diseases involving impairments in emotional learning. Despite the importance of mAChRs in the physiology and pathophysiology of emotional memory, there have been no studies that have systematically examined the molecular, cellular, and network-level mechanisms by which mAChRs regulate BL function. The studies outlined in this proposal will combine multiple-labeling electron microscopy and multiple-labeling confocal immunofluorescence with state-of-the-art electrophysiology and optogenetics to address this significant knowledge gap. Our long term goal is to understand how muscarinic receptor regulation of the amygdala can be manipulated for therapeutic purposes. The objective here is to determine how synaptically released acetylcholine, acting on mAChRs, regulates neuronal activity in the BL. Our central hypothesis is that distinct mAChRs on different neuronal subpopulations play discrete roles in regulating neuronal oscillations, filtering salient signals and strengthening glutamatergic inputs in the BL. Our hypothesis is based on preliminary data which reveal that mAChRs strongly modulate BL circuits in a manner distinct from that reported in other brain regions. This hypothesis will be tested by pursuing three specific aims: 1) To define muscarinic modulation of neuronal excitability and oscillatory activity at pyramidal cells and interneurons in the BL; 2) To define frequency-dependent gating of glutamatergic and GABAergic transmission by presynaptic mAChRs; and 3) To determine the functional effect of mAChRs on synaptic transmission and plasticity in the BL. This project encompasses several of the priorities/themes of the BRAIN Initiative, including characterizing cell types in the nervous system, developing tools to manipulate these precisely defined neurons, creating structural maps of the brain, and crossing boundaries in interdisciplinary collaborations. Information about mAChR regulation of BL circuits will be critical for the development of therapies to ameliorate severe neuropsychiatric disorders, treat drug addiction and diminish the emotional deficits produced by Alzheimer's disease.